地下水学会誌 第 54 巻第 4 号 191 ～ 206（2012） Technical Report Characteristics of the chemical composition of groundwater on Tanegashima, Yakushima, and Nakanoshima islands, Kagoshima Prefecture, Japan Seizen AGATA＊, Maki ISHIKI＊＊, Hideaki SAKIHAMA＊＊, Akira TOKUYAMA＊＊＊, Hiroshi SATAKE＊＊＊＊† and Jing ZHANG＊＊＊＊ Abstract Twenty-six groundwater samples were collected From Tanegashima, Yakushima, and Nakanoshi- ma islands between July 27 and August 2, 2003. The interpretation of chemical analyses suggested that the groundwater on the islands have been affected by sea salt (ss). Chlorine (Cl－) concentrations on Yakushima, which were inversely related to elevation, were considerably lower than those on Na- kanoshima and Tanegashima, reflecting higher annual rainfall on Yakushima. However, groundwater samples from Yakushima had a higher ratio of ss components—more than 50% (except one sample). Non-sea salt (nss) components of the groundwater on Tanegashima, Yakushima, and Nakanoshima were found to be affected mostly by the chemical weathering of plagioclase feldspars. The groundwa- ter on Tanegashima was also thought to be affected by ion exchange between clay minerals and Ca2＋. The effects of sulfuric acid and carbonic acid on chemical weathering were nearly equivalent on Na- 2－ kanoshima. Therefore, the average concentrations of nssSO4 and dissolved SiO2 were remarkably higher than those for Yakushima and Tanegashima. The arithmetic mean concentration of nssK＋ on Yakushima was high even though the total concentration of all nss components was low. The nssK＋ probably originates from chemical weathering of K-feldspar in granite. The total concentration of nss components on Yakushima was remarkably lower than that on Tanegashima or Nakanoshima. Low － NO3 -N concentrations in these mountainous and forested islands reflect a low load from anthropogenic activities. Key Words: groundwater chemistry, chemical composition, chemical weathering, nitrate, Tanegashi- ma, Yakushima, Nakanoshima 1. Introduction islands, with population sizes of 35695, 13875, and 183 in 2000, respectively, are located between Ky- Tanegashima, Yakushima, and Nakanoshima ushu and Amamioshima islands in southwestern ＊ Yaeyama High School, 275 Tonoshiro, Ishigaki-shi, Okinawa 907-0004, Japan ＊＊ Graduate School of Engineering and Science, University of the Ryukyus, 1 Senbaru, Nishihara-cho, Okinawa 903-0213, Japan ＊＊＊ Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishi- hara-cho, Okinawa 903-0213, Japan ＊＊＊＊ Earth and Environmental System, Graduate School of Science and Engineering for Research, University of Toyama, 3190 Gohuku, Toyama-shi, Toyama 930-8555, Japan † the late doctor ― 191 ― 地下水学会誌 第 54 巻第 4 号 191 ～ 206（2012） Japan, as shown in Figure 1 (Kagoshima Prefec- ter and the air mass on Yakushima, which is locat- ture, 2001; Toshima Village, 2001). These islands ed on the axis of the Kuroshio Current, are greater are in a subtropical climate region. Tanegashima than those for the other Nansei Islands. Therefore, Island has an area of 454 km2 and a maximum el- climate disturbance (i.e., rainfall) is strongly af- evation of 282 m. Mt. Miyanouradake (1935 m fected by low-pressure systems passing through a.s.l.) on Yakushima Island is the highest mountain the area south of Yakushima (Ishijima and Itokazu, in Kyushu, but many mountains on this small is- 1980; Eguchi, 1984). land (503 km2) are higher than 1500 m. Therefore, Nagafuchi et al. (2001) studied the compo- Yakushima Island has both temperate and subtrop- sition of rime ice on Yakushima. The geochemi- ical climate zones. Nakanoshima Island, a volcanic cal characteristics of acidic stream water and the island, has an area of 34.5 km2. Mt. Otake (979 m effects of acid deposition on the water quality of a.s.l.) is its highest mountain. The annual average mountainous streams were studied on Yakushima temperatures on these islands are 19.6 ℃, 19.2 ℃, by Nakano (2001) and Nagafuchi et al. (2003), re- and 18.9℃, and the annual amounts of rainfall are spectively. Yamanaka et al. (2005) described river 2322 mm, 4359 mm, and 3082 mm (JMA, 1971- and spring water chemistry on Yakushima. These 2000, 2003-2006), respectively. A warm current studies, with respect to the effect of acid deposi- (Kuroshio) in the East China Sea flows northward tion on the stream water of Yakushima Island, re- west of Nakanoshima and passes eastward south of vealed that the acid species are transported from Yakushima and Tanegashima to the Pacific Ocean. the Asian continent. The results show that the pol- Rainfall on Yakushima is much greater than that on lutants that acidify stream water on Yakushima Is- Tanegashima. Differences in temperature between land accelerate chemical weathering, mainly of pla- the surface seawater and the atmosphere and of gioclase. water vapor pressure between the surface seawa- Many chemical studies on meteoric water have been conducted on Yakushima Island, mainly because the island was registered as a UNESCO World Natural Heritage Site in 1993. However, the meteoric waters of Tanegashima and Nakanoshi- ma islands have yet to be studied. In this study, contributions from sea salt and chemical weather- ing of silicate rocks inferred from chemical data of groundwater will be presented and discussed to clarify the geochemical characteristics of ground- water on Tanegashima, Yakushima, and Na- kanoshima, each of which has different geographi- cal and geological environments—sedimentary rocks, granites, and volcanic rocks, respectively. 2. Geological Features Tanegashima is elevated from the main sub- Fig. 1 Locations of Tanegashima, Yakushima, and marine bedrock, which consists of Kumage Group Nakanoshima islands in Kagoshima Prefec- formations including the Sumiyoshi, Fukago, ture. Hamatsuwaki, and Nijyuuban formations (Figure ― 192 ― 地下水学会誌 第 54 巻第 4 号 191 ～ 206（2012） 2). The formations consist of sandstone, shale, and Tiba (1990) reported that andesitic to dacitic vol- tuff-silt rocks that represent neritic marine depos- canic rocks on Nakanoshima contain plagioclase, its. These rocks are widely distributed around hills orthopyroxene, cliopyroxene, and amphibole. The at about 200 m (a.s.l.) in the northern, central, and island has hot springs in Satomura (Nishiku and southwestern parts of this island. Eroded surfaces Amadomari hot springs) and Funagura (Higashiku of Kumage Group formations are partially covered Hot Spring) (Daishi, 1985). unconformably by Kukinaga Group formations. Unconformity surfaces of Kumage and Kukinaga 3. Experimental Methods Group formations are covered by Masuda Forma- tion sandstone, which is distributed below 120 m Thirt een, eight, and five groundwater samples (a.s.l.) in the southern part of the island. Addition- from Tanegashima, Yakushima, and Nakanoshi- ally, unconformity surfaces of Kumage and Kuki- ma islands, respectively, were collected into 0.5-L naga Group formations and the Masuda Formation polyethylene bottles between July 27 and August are overlaid by the Hase Formation, which consists 2, 2003. Water temperature, pH, and EC were mea- of gravels distributed in the southern part of the is- sured at each sampling point using a glass elec- land (Hayasaka, 1985). A camptonite dike about 20 trode pH meter (HM-10P; Toa Corp.) and an elec- km long exists in the northern part of this island, trical conductivity meter (CM-10P; Toa Corp.). as presented in Figure 2. Minerals in the intrusive After filtration with a 0.45-µm membrane filter, Na＋, ＋ 2＋ 2＋ － 2－ － rock include biotite, alkali amphibole, and plagio- K , Ca , Mg , Cl , SO4 , and NO3 concentra- clase (albite to oligoclase), which is associated with tions were determined in the laboratory using an calcite (Hayasaka, 1985). Yagi et al. (1975) report- ion chromatograph (L-7000; Hitachi Ltd.). The － ed that this intrusive rock is older than the Kukina- HCO3 concentrations were determined by alkalin- ga Group, which includes pebbles and boulders of ity titration (JISC, 1992) and dissolved silica by the the camptonite dike. Tanegashima is cov ered with molybdate yellow method using a spectrophotom- andesitic two-pyroxene volcanic ash (Hamazaki, eter (AE-300; Erma Inc.; JISC, 1992). 1972). The volcanic ash soils consist mainly of hy- persthene, augite, basic plagioclase, and an uniden- 4. Results and Discussion tified iron mineral. Clay minerals in the soils in- clude mostly allophane, chlorite, vermiculite, illite, Table 1 presents the chemical composition of and kaolinite, among others. groundwater from Tanegashima, Yakushima, and Figure 3 shows granite in the central part of Nakanoshima islands. Ion charge balances (Σcation Yakushima Island that is surrounded by sedimen- - Σanion)/(Σcation + Σanion) for 23 of 26 samples tary rocks, including siltstones, sandstones, and were in the ± 5% range. The other three samples gravels (Hayasaka, 1985). Major minerals in the (Nos. 2, 3, and 4) on Tanegashima had balances granite are biotite, quartz, K-feldspar, and plagio- ranging from － 5.1% to － 6.3%. The non-sea salt clase feldspar. Argillaceous rock contains biotite, (nss) component concentrations shown in Table 1 muscovite, chlorite, and others (Hayasaka, 1985). were calculated using the following equation (Ishiki On Nakanoshima Island, Otake and Shizaki et al., 2009): volcanic rocks, consisting of either lavas or tuff nssX=X -(X /Cl ) × Cl （1） breccias, are distributed in the northern and the groundwater sea sea groundwater southern halves of the island, respectively (Fig- where X is